THURSDAY, March 9, 2017 (HealthDay News) -- A new study shows promise for an oral vaccine to protect wild apes against the Ebola virus.

The virus is a real threat to Africa's great apes -- for example, Ebola is estimated to have killed one in three wild gorillas in recent decades.

However, the researchers noted that the new vaccine research was stopped early due to U.S. laws regarding medical testing in chimps.

In the current study, six chimps received the oral vaccine while four were injected and served as a control group. The animals all showed a strong immune response after 28 days. The researchers said there were no side effects.

The oral vaccine could potentially be administered to wild apes through food.

"In 2014, the world was gripped by fears of an Ebola virus pandemic. Yet few people realize that Ebola has already inflicted pandemic scale mortality on our closest relatives," said lead researcher Peter Walsh from the University of Cambridge in England.

"African apes are also threatened by naturally occurring pathogens like anthrax, and the increasing overspill of human pathogens, such as measles. A glimmer of hope lies in the fact that many of the disease threats are now vaccine-preventable," Walsh said in a university news release.

But researchers are raising alarm because it's becoming more difficult to find captive chimpanzees in the United States to take part in research projects.

"We have developed a very promising tool for inoculating ape species against the myriad deadly diseases they face in the wild, but continued progress relies on access to a small number of captive animals," Walsh said.

The United States was the last developed country that allowed medical testing on chimps. The scientists behind the new study explained that the new regulations on chimp research will, ironically, be harmful to wild apes because any vaccines that protect them must be tested in captive animals.

The researchers said the obstacles are so great that the new vaccine may not progress enough through research to actually reach wild apes.

"This may be the final vaccine trial on captive chimpanzees: a serious setback for efforts to protect our closest relatives from the pathogens that push them ever closer to extinction in the wild," Walsh said.

"Oral vaccines offer a real opportunity to slow this decline," Walsh added. "The major ethical debt we owe is not to a few captive animals, but to the survival of an entire species we are destroying in the wild: our closest relatives."